Not applicable.
This invention relates generally to antennas and more particularly to an improved circular polarized antenna with large scan angles.
In antenna applications, it is often desirable to transmit and receive signals using circular polarization. One common technique to transmit and receive circular polarization is to use a pair of cross dipoles and by feeding a signal in phase quadrature i.e., a 90 degrees phase difference introduced between each element and its orthogonal counterpart a circular polarized signal is produced. Although a true circular polarized signal is approximated on boresight of the antenna, as signals are transmitted and received off boresight, the signal less approximates a circular polarized signal resulting in a larger VSWR.
In array antennas, the need for scan independent arrays has been recognized. It is desirable to maintain circular polarization over a large scan volume as well as constant scan impedance. In order to receive maximum power, the antenna load must be conjugate matched to the antenna scan impedance. Any variation in the scan impedance results in a mismatch between it and the load, whereby a portion of the received energy is reradiated instead of being absorbed by the load. The greater the mismatch, the larger this portion becomes. The need for this additional constraint arises when considering an array for satellite communication purposes. Antennas mounted on satellites typically transmit and receive circular polarization since their location relative to the ground-based antenna is constantly changing. Furthermore, problems resulting from Faraday rotation can also be avoided by using circular polarization.
It would, therefore, be desirable to provide an array antenna with improved scan impedance and radiation field properties when transmitting and receiving circular polarization.
In accordance with the present invention, an antenna includes a first dielectric slab having a first surface and a second opposing surface; a ground plane disposed over the second opposing surface of the first dielectric slab; a first array of antenna elements having a first polarization characteristic, said first array disposed in said dielectric slab and spaced from said ground plane by a first distance; a second array of antenna elements having a second polarization characteristic, said second array disposed in said dielectric slab and spaced from said ground plane by a second distance, wherein said second array is disposed such that the polarization characteristic of said second array is orthogonal to the polarization characteristic of said first antenna; and at least one additional dielectric slab disposed over the first surface of the first dielectric slab to compensate for variation of antenna impedance with differing scan angle. With such an arrangement, a circular polarized antenna array is provided with a scan impedance that has a minimum variation over a large scan sector.
In accordance with a further aspect of the present invention, an antenna includes first and second arrays spaced apart by a first dielectric slab having a predetermined thickness, each of said first and second arrays provided from antenna elements having a linear polarization with the antenna elements in said first array disposed orthogonally to the antenna elements in said second array. The antenna further includes a feed circuit coupled to said first and second antenna arrays to provide said first and second antenna arrays having a phase relationship such that the antenna receives and transmits signals having circular polarization and dielectric material having differing characteristics for providing an impedance transformation that varies with scan angle in accordance with the a scan impedance of said antenna thereby providing the antenna having a scan impedance which is substantially the same over a scan sector while at the same time preserving the response characteristic of the antenna to signals having circular polarization. With such an arrangement the impedance of the transmitter/receiver can be essentially conjugate matched to the array scan impedance across a large scan volume, resulting in maximum transmitted/received power. In addition, such an arrangement also maintains near perfect circular polarization over this same scan sector, any deviation from which would produce additional power loss in a transmit/receive signal.